FIG. 1 shows a structural schematic view of a cellular radio communication system. As shown in FIG. 1, the cellular radio communication system mainly includes a terminal, an access network and a core network. The network formed by a base station or by a base station and a base controller is called a Radio Access Network (RAN) which is responsible for access layer service, is such as a radio resource management. Physical or logical connection may exist between base stations, such as a base station 1 and a base station 2 or a base station 3 in FIG. 1, according to the practical situation. Each base station can be connected with one or more Core Network (CN) nodes. The core network is responsible for non-access layer service, such as position updating, and is an anchor point of a user plane. User Equipment (UE) may be various types of equipment capable of communicating with a cellular radio communication network, such as a mobile phone and a notebook.
The cellular radio communication system is divided into a Frequency Division Dual (FDD) system and a TDD system by modes. Taking Long Term Evolution (LTE) as an example, uplink signals and downlink signals of the FDD system are transmitted on different frequency bands, while the uplink signals and the downlink signals of the TDD system are transmitted on the same frequency band. In a TDD system, uplink and downlink multiplex the same frequency band in a time division multiplexing mode. Because of the same-frequency time division multiplexing relationship between the uplink and the downlink of the TDD system, different cells of the TDD system need to be kept synchronous on an air interface; otherwise, mutual interference may be introduced when the uplink transmission and the downlink transmission of different cells are overlaid in time.
The air interface synchronization of the existing TDD system can be realized by global synchronization signals of a Global Positioning System (GPS) and an IEEE 1588v2 network synchronization protocol. The GPS synchronization may have very high precision, and has been widely used in a Time Division-Synchronous Code Division Multiple Access (TD-SCMDA) system. However, it is of high cost and cannot be used in the places which cannot receive GPS signals, for example, indoor places. The IEEE 1588v2 protocol can implement synchronization based on a terrestrial IP protocol. However, it has low synchronization precision and high requirements on the is time delay of a transmission network. It is difficult for the two synchronization solutions to meet using requirements in a home base station scene of the TDD system. In order to overcome the defects of the two solutions, another air interface monitoring-based synchronization solution is provided.
FIG. 2 shows a schematic view of synchronizing an unsynchronized base station to a synchronized base station. As shown in FIG. 2, an unsynchronized TDD base station (a TDD base station which is powered on just now, for example) searches cell signals via an air interface after being powered on, when signals of a synchronized cell(a macro cell of the TDD, for example, which usually adopts the GPS synchronization and the synchronization precision is highest) is received, an unsynchronized cell implements synchronization of the air interface by receiving and aligning the downlink synchronization signals of the synchronous cell. FIG. 3 shows a schematic view that the unsynchronized base station is synchronized through a multi-hop TDD air interface. As shown in FIG. 3, the synchronized base station 2 implements synchronization with the synchronized base station 1 according to air interface synchronization signals; and then, the unsynchronized base station and the synchronized base station 2 implement synchronization. Along with the increasing number of hops, the synchronization precision will decrease. Therefore, the synchronized base station with fewer hops is selected as far as possible when the unsynchronized base station selects a cell to be synchronized with. Therefore, based on this, each synchronized base station shall have a synchronization level n (the lower the level, the higher the precision; or the higher the level, the higher the precision), the synchronization level of a higher-level synchronized base station shall be level (n−1) and the synchronization level of a lower-level synchronized base station shall be level (n+1). The base station with the highest synchronization level is the TDD base station which adopts other high-precision synchronization, such as GPS synchronization, of non-air interface synchronization. When the unsynchronized base station receives signals from two different base stations, the base station with the highest synchronization level is selected according to the synchronization level.
In an air interface monitoring synchronization solution, the unsynchronized base station needs to be able to receive the synchronization signals of the synchronized cell and knows the synchronization status (being synchronous or not) and the synchronization level so as to select the synchronized base station with a higher synchronization level to achieve a better synchronization.
In the existing solution, a synchronized cell broadcasts the synchronization status and the synchronization level, and the unsynchronized cell reads the broadcasting of the synchronized cell so as to acquire the synchronization status and the synchronization level of the synchronized cell.
The solution has the following defects:
1) the overhead of the air interface increases due to the adding of the broadcasting information on the air interface;
2) the service interruption time of the air interface will increase due to the reading of the broadcasting message by the base station, since a base station cannot send signals for a UE serving for the present base station when the base station reads signals sent by another base station through the air interface. Because of the change of network topology, in particular the frequently switching of the home base station, the TDD base station needs to monitor the synchronization status and the synchronization level of the synchronized base station thereof in real time so as to implement a timely update; therefore, the service of the cell will be influenced frequently by interrupting the service to read the broadcasting message each time;
3) based on the consideration of safety and reliability, for an illegal base station or a base station which is set incorrectly, the unsynchronized base station monitoring these base stations cannot acknowledge the reliability of the broadcasting information acquired by the unsynchronized base station; and
4) based on the consideration of compatibility, for an old TDD base station or a base station of which the information cannot be broadcast by the air interface, the unsynchronized base station cannot implement synchronization with respect to the base stations.